1. Field of the Invention
The invention relates to a method of controlling a motor of a battery-operated power tool, such as a hand-held circular saw and to a control device for carrying out the method.
2. Description of the Prior Art
A motor drives a power tool in an “on” position of a motor switch that is actuated manually by the user. In battery-operated devices, as a rule, the electronics in their entirety are also disconnected from the battery by the motor switch in order to prevent a discharge of the battery when not in use. Further, in order to prevent damage to the motor or electric components, the process for controlling the motor of the power tool is generally designed in such a way that the control electronic switch off the power tool and motor when a certain supply voltage is not available. The undervoltage lockout is carried out, for example, when the battery is discharged or is defective.
An undervoltage lockout of the type mentioned above is known, e.g., from DE 36 11 884 A1 in a battery-operated hand tool. A lockout relay controlled by a flip-flop circuit is provided for carrying out the control process, the flip-flop circuit being controllable by an undervoltage monitor. The cutoff relay can be connected to voltage by means of a main switch to start operation of the power tool. The cutoff relay has a switch that short-circuits the motor in case of failure of the cutoff relay so that the motor is braked when the main switch is released again or when the undervoltage monitor causes an undervoltage lockout by means of the flip-flop circuit.
It is disadvantageous that it is impossible to distinguish a cutoff caused by actuation of the motor switch from a cutoff caused by inadequate power supply, e.g., when the battery is discharged or defective. Further, it is not possible to switch the power tool back on immediately after the switch has been turned off when the control electronics have a microcontroller, as is commonly the case in modern power tools, because, while the voltage has fallen below the threshold for undervoltage lockout, the voltage has not yet fallen below the threshold for reinitializing the control electronics and the microcontroller. This is the case particularly when the undervoltage lockout responds very quickly, i.e., has only a short time window before reacting (cutoff). Depending on the design and construction of the electronics, the time window can last up to several seconds before it is possible to turn the power tool back on again by means of the motor switch, which is annoying for the user. Different control options are not possible.
It is the object of the present invention to provide a control method process which avoids the above-mentioned disadvantages.